James Gurney

This daily weblog by Dinotopia creator James Gurney is for illustrators, plein-air painters, sketchers, comic artists, animators, art students, and writers. You'll find practical studio tips, insights into the making of the Dinotopia books, and first-hand reports from art schools and museums.

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Thursday, March 11, 2010

The great color theorist Albert Munsell observed that a given color reaches its greatest chroma at one particular value, called the home value or the peak saturation value. That peak value varies from color to color. An orange-yellow, for example, is most intense at a very light value, while blue is strongest when it is very dark. Red reaches maximum chroma at a middle value.

The hand-painted chart takes the three hues through all possible degrees of chroma and value. Yellow becomes a brown when it darkens, and you just can't get a strong chroma from a dark yellow.

In the chart, chroma is meant to be constant along a vertical line, while value is constant on a horizontal line. It’s good color mixing practice to make these charts, and Munsell-schooled artists benefit greatly from the experience.

For these charts I have arbitrarily limited the chroma range to six steps. But in Munsell’s system, there can be many more steps, and theoretically there’s no upper limit to the measurement of chroma. New pigments keep pushing the range farther out.

This digital chart from David Briggs's excellent website huevaluechroma shows a range of hues taken through the paces. Follow that link to his discussion of the same topic.

A pure single-wavelength colour, as produced by a laser, is the upper limit. If you tried to reproduce this in paint, you would find that as the waveband narrows, the proportion of light relected gets less - i.e. the surface gets darker.

However, if you shine a very bright light onto a narrow-band paint, you can see its colour.

Jim, you are contradicting your previous statement, since, as you pointed out, not only there are limits but they differ for each hue and value. What you meant is perhaps that the Munsell scale is not limited to current available pigments but can be extended to any future ones - precisely because it can be extended *to* the theoretical limit. It is a perceptual scale that represents all colours, not just a fixed gammut generated by an arbitrary set of so-called "primary" colors.

"A pure single-wavelength colour, as produced by a laser, is the upper limit"

Don, I don't think that is right, actually. If you have a surface that reflects only in the yellow wavelength, it will seem rather dark for it wastes most of the illuminant, and, by seeming dark, it will have low chroma (so you'll end up with that sort of brown or greenish thingy that you can see in Gurney's low chroma yellow, in the post). A colour may have maximum saturation, but low chroma, since chroma depends on saturation and value.A very chromatic yellow paint will in fact usually reflect a lot in the red frequencies. In order for the hue of a colour to be yellow all that you need to ensure is that the weighted sum of the frequencies is a yellow hue, not that only the yellow frequency is reflected. An adequate continuous reflective spectrum that extends to the reds will still have yellow hue but it will have the necessary lightness to be a high chroma yellow (it is a balancing game between a certain loss of saturation and a gain in lightness).

To put it another way, perceived "purity" of a colour is not determined by its saturation but by its chroma, which is saturation times lightness. Hence the balancing act.

This is very badly written, but I am in a hurry. I hope I haven't made errors of fact, it's very easy to do so when talking color.

One of the problems of discussing the science of color is the imprecision of terms. Terms that mean one thing to one person, or scientific perspective, might mean something very different in another. Chroma and saturation are examples of two such words.

My perspective and learning comes from the Munsell texts. Of the three axes of color, hue (wavelength) and value (percentage of reflected light) are the most scientific, but chroma is perceptual; it is not the result of a mathematical process, but derived from perceptive differences as reported by sampling the opinions of about a million people in the early 1900s (if I'm recalling the story correctly).

In Munsell, then, chroma is not "saturation times lightness". It is related to both saturation and lightness, but it is not a direct formula.

Generally, in Munsell terms, saturation isn't used much, but may be more properly understood in the way a chemist might use it: the point at which a given medium can't hold any more of a substance, like sugar in water. In oil paint, it's pigment and binders. A color like (say) lamp black, nearly neutral, can be fully saturated (full of pigment) with an effective chroma of zero.

Lightness has to do with how much reflected light is allowed into the visual cortex by the sensitivities of the cones in the eyes. We perceive yellows and oranges as "light" colors, but there's nothing inherently light about those wavelengths in terms of their physical properties; the aggregate bell-curve of our eyes' sensitivity to hues is just at its peak at that range of the spectrum. We let in more of that color light, so we perceive the colors as "light." If our sensitivity curves were different, we might perceive purples and blues as light colors, and yellows and oranges as dark. There's so much room at the bottom of the yellow-orange bell curve that we give distinct names to the low-value colors because they're so far from the high-value ones: brown and olive green. However, the hues are still orange and yellow, respectively.

The most chromatic pigments available right now, as Margaret Livingstone explains in her book, are the "day-glo" pigments, whose chromas haven't been measured to my knowledge, but I would estimate them to exceed 20 (the highest chroma chips in the Munsell Glossy Book is 16), maybe 22. They work by actually absorbing high-frequency wavelengths (like blue) and converting them into lower-frequency wavelengths (like yellow/orange/red). This can make the material emit more wavelengths than it reflects, which gives it its extraordinary chroma. However, it's not lightfast, so it can't be used in anything that needs to last very long, and certainly not in oil painting.

We could go on about this at great length, but the blogger comment format is a bit limited and it's probably beyond the scope of what James intended with his posts. I'd strongly recommend that anyone interested in the Munsell system and color theory join Graydon Parrish's Rational Painting forum, where it's one of the main subjects of discussion.

Fun post--I like the hand-painted charts. My husband does some similar exercises with his color & design classes. There's even an ancient Munsell color wheel in his classroom, or tree, or whatever it's called--it's a plastic contraption with "leaves" that rotate around a central axis.

In any case, for me it was a useful exercise to mix the colors of differing hue, value, and saturation, as someone whose art training was much more conceptual and concentrated less on the basics.

I agree that it's easy to misunderstand terms in color theory, we have to be specific. I didn't mean saturation in that way (as in "this brush is saturated with paint"), but as it is defined, say, in the CIE diagram, which is also the sense that Don referred to. What Don said is, I think, perfectly correct when talking about saturation, my point was just that Jim wasn't talking about saturation but about chroma, and not about emmited lights but about reflective surfaces under a given illuminant, so, in my view, Don was correct but beyond the point.

Also, you point out that chroma is perceptual. That is correct, and in fact, in Munsell terms, not only chroma but also value and hue are perceptual. All are ordered by asking observers to set up "minimal difference perceptual scales", as far as I know (I am not fully sure since it's hard to find the details on how the Munsell scale was actually made, since you studied Munsell I'd appreciate if you could clarify further).

Now, given that all the scale is perceptual, there is still no contradiction, once it is built, in relating it to the other scales, perceptual or not, like CIE hue-saturation-lightness or anything else. A mathematical function does exist, and you can convert value to lightness, etc. What you find is that if you try to measure chroma, as defined by munsell, on the CIE space, you'll find that chroma depends both on lightness and saturation, growing with both. So, a paint of yellow hue and maximum saturation will not be high chroma unless it also has high enough lightness.

Cripes. What I said before wasn't strictly right. Best at least be correct when you're being pedantic.

>Don Cox-A laser isn't generally completely monochromatic because you cant control energy levels of photons with 100% accuracy for 100% of those which are created to make the beam. Even so, a monochromatic light has a wavelength which can be adjusted with accuracy only limited by quantum mechanical stuff that isn't really necessary to go into. Suffice to say, its close enough to limitless that people only bothered to figured out why it wasn't in the last hundred years.

The reality of things is that when you see light, you see a bunch of photons reacting with your retina, each with exact and quantised colours but also with a combined composite colour; the one you perceive when 100 000 of them hit the same spot inside a second. if 99 999 of them have a wavelength of 630nm and one of them has a wavelength of 627nm, the colour composite you see will be imperceptibly below 630nm. That goes the same for the emitted and reflected colour sets, as only the additive properties of the pigments change between them.Saying then, that there is a limit to the number of attainable colours is kind of like saying that there is a limit to the number of songs that will be written before we're just writing a song that has already been crafted before. You can vary the length, timing, scale, chords, instruments, lyrics and post production effects in such a way that, whilst that remains true, its beyond the realm of plausibility.

> Antonio:"Also, you point out that chroma is perceptual. That is correct, and in fact, in Munsell terms, not only chroma but also value and hue are perceptual. All are ordered by asking observers to set up "minimal difference perceptual scales", as far as I know (I am not fully sure since it's hard to find the details on how the Munsell scale was actually made, since you studied Munsell I'd appreciate if you could clarify further)."

Antonio, most of what I know about Munsell came from the Munsell Student Book, available from X-rite for about $70 (and which can generally be found cheaper on ebay/etc with a little digging). It's a complete history and explanation of the Munsell system; I can't recommend it enough. It's extremely thorough and well-written.

My understanding is that the spacing of the hues has perceptual intervals, but hues themselves are wavelengths, of course, and determined by spectrophotometry. There is also a chart in that book that shows the reflectance percentages for value, which is consistent across all hues and chromas: a value 7 surface always reflects exactly X.XX% of visible light (don't have the exact numbers right at hand). It may be that they, like hue, were spaced peceptually initially, but there are hard-and-fast numbers now which govern them; that's what I meant to say earlier.

Saturation and lightness as terms don't come into play much in Munsell terminology; Hue, value and chroma (or HVC for short) is the main lingo for describing and working with color. I don't understand as much about saturation and lightness as I would like to, and I'm digging for more in Livingstone's excellent book (which I can only work through so fast while keeping my head from exploding), and I'm also working through David Brigg's excellent site which is another rich encyclopedia of knowledge; he's forgotten more about this stuff than most of us know. :)

It's easy to misspeak on a subject so complex, so I hope we can all be friendly with our discourse and corrections of errors; I'll thank anyone who spots and corrects mine.

I'm a big fan of the Briggs site, it's the best resource on colour I know after Macevoy's handprint site (which I cannot praise enough). Thanks for sharing, regarding the references and details on Munsell.

I've been looking for a info on the Munsell system's details for some time (on how exactly it was setup, not on how to use it after it was setup, since that is readily available). It seems there ia a paper out there that might contain that info, but my university doesn't have it and I cannot find it anywhere in Portugal. By the abstract, it seems someone had the same nagging questions I do, and actually managed to answer them.

I leave the reference and abstract below. If somebody has read it please speak up and share what you learned :)

Here it is:

Development of the 1929 munsell book of color: A historical review Roy S. Berns, Fred W. Billmeyer Jr.

Color Research & ApplicationVolume 10 Issue 4, Pages 246 - 250

Abstract Despite worldwide acceptance and usage of the Munsell color order system, accurate documentation of the experimental conditions associated with the original visual scaling experiments has not existed in the archival literature. Discussion with several coworkers involved in these scaling experiments has enabled us to provide a substantial portion of this documentation.

My problem with the usual explanations of the munsell system stems from this:

Munsell's is a perceptual scale. Without discussing what perceptual tests we are using it is meaningless. We state that there are three dimensions: value, chroma, hue. But if we don't define very clearly how we measure them perceptually, then those are only names.

For instance: how did Jim know what colors he should place alongside each gray patch on the scale on this post? Clearly he has some notion of what the value of a non-gray colour means. And how did he know that that brownish thing at low chroma/value was still a yellow hue? Most people wouldn't say so if it was presented to them independently (they'd guess a green hue). Clearly he has some operational/perceptual notion of what a constant hue is.

But those notions are not clear in his explanation of Munsell's scale. They have not been stated. And they are *the* most important part of the deal.

And that's no fault of Jim, it is very hard to find a good explanation of this even in sites and books who dedicate a lot of effort to explaining Munsell. It is as if the whole question is not acknowledged at all. Yet I am sure that the people who built the scale spent a lot of thought on that.

I think I can give a proper explanation of how it can be done, but I cannot be sure of how it actually was/is done unless I find it explained somewhere.

Antonio, I'm not sure if this is what you're getting at, but the Munsell Glossy Book of Color is a 1600-chip volume of references for Munsell colors that provide concrete definitions of what each chip is, and this is the reference that many of us use. Unfortunately, it is hideously expensive (around $800 now, I think), but it does have very high manufacturing costs.

There is a poster version of the set made by BabelColor which can be had in the $50 range, but it has limitations on accuracy which are explained on their site.

We know that colors in the olive-green range are actually dark yellows because we can measure the light reflected from them with a spectrophotometer. If the wavelengths are in the yellow range of the spectrum, then it's yellow, even if our eyes and brain perceive them as green. The mixing properties will governed by the actual hue, not the perceptive hue. And, as I said earlier, value is also a concrete scientific term, with concrete reflectivity steps. Chroma is more complicated, but hue and value are pretty straightforward.

The x-rite ColorMunki spectrophotometer, which I have, "speaks" L*a*b natively, and has software to convert the L*a*b values to Munsell HVC by comparing them against a database. I re-calibrated mine to my copy of the "Big Book" just for increased local accuracy and to correct a few typos in X-rite's database. I can, and do, use this to verify control of colors, particularly the near-neutrals, which can be difficult to work with.

It boils down to the fact that you need some reference standard to compare against. Munsell's big effort was building that reference, and that's what the book of chips is, and the accompanying text in the (much cheaper) student book. I wish the gear could be cheaper, but the fact that it isn't doesn't detract from the value or accuracy of the Munsell system.

I think, again, that you will find all of your questions answered in the Student Book, which can be found relatively inexpensively. It's extremely scientific, specific, and lucid about all of this.

"I realise I am being too vague myself, and this probably cannot be understood. I'd better shut up until I can write this coherently" :)

I understand what you are saying, and you are not at fault in any way for not getting where I am driving at, its my fault for not being explicit. I'll try to write some coherent explanation of what I mean and maybe make it a post on my own blog or something.

Thanks for the references again, I've heard of the legendary Big Book but never saw one of those mythic beasts in the flesh. :)

And I'll look up the spectrophotometer you mentioned, I haven't seen one since physics class :), I think you may be responsible for me breaking the bank with a new toy!

My guess is that because color, and its application and definition(s), is so basic and fundamental that it is like a discussion on politics or religion. In the same way that our feelings about these subjects is intensely personal, there seems to be some deeply rooted thoughts about color.

I suppose that each person's color experience is entirely unique and therefore this category starts to fall under the "what is art" and "what is a sellout" kind of discussion.

If this was a "what is art" sort of business we'd end up insulting each other and making a mess of the premises, and no progress would ever be made. In this case, instead, we can have civil disagreements that actually have a chance of getting resolved by statements of fact, and we may actually inch forward a bit.

Color is really complicated. You have to deal with the physics part (what light is and what it does), the physiological part (how it interacts with your eye and brain), with the theoretical part (how to organize all the knowledge regarding the previous two part into a coherent body, which requires mathematics, physics, experimental setups, conventions, even commissions (like CIE or the Munsell organization) and decades long processes, sometimes not very well documented). And connecting all this stuff requires some philosophical thinking, which in these days we are not very used to, philosophy being mostly taught as a distraction in school. We can have phenomenological theories, we can haver perceptual theories, we can have theories of all sorts, and its not just about choosing the right one, its about knowing what each one is and how it helps us (of course, a few are just wrong and useless, but not only one useful one remains after descarding those). For instance, CIE made a very precise theory, but it did not fit well with some perceptual rquirements. Munsell had a purely perceptual, but then it had inchoherences, for perceptual measurements are far vaguer. Then with the so-called renotation (which Slinberg was just referring to in his post), they sort of joined efforts to regulate the incoherences with Munsell theory, but then it is debatable that the result is actually some other type of theory (having changed the means of construction), and there is still place for other ones. For instance, it is good to have a theory that is purely perceptual and operational, in the sense that I don't need to use a spectrophotometer but only my eyes, even if that theory is limited in scope and precision (different tools for different peuposes - every theory is another set of glasses to peer through at the same object, and see it in a different light).

Then, of course, after dealing with all this, presuming you are interested in color because of art, you still have to deal with actually getting the colors from paint mixing! And then, may the Lord protect us all! :)

I appreciate your comments would agree with pretty much everything that you said. I have seen these comments be a little more contentious:http://gurneyjourney.blogspot.com/2010/02/color-wheel-part-4.html

There are definitely some hurt feelings (or riled emotions) in some of those comments. It's interesting to me how much this topic fires people up. Personally, I think that it's great have that kind of passion for art =)

Color Theory stunned me in college. I thought it was all about how to make a pretty picture and define our palates but man-o-man they had to get all Munsell on us! And that was just the beginning. Frankly, I find these threads so informative for a post graduate and miss those heated classroom debates! /tear

I have a copy of the Berns and Billmeyer paper and would be happy to send you a scan of it - just drop me an email (address on my website). Basically they say that the people involved in the 1929 revision were such that they would have known what they were doing, but unfortunately we don't know what they did, apart from how they derived the value scale, which was documented in publications. In particular, the basis for selecting the principal hues is a complete mystery. Fortunately the 1943 renotation, which is the basis of the current Munsell book, has been thoroughly documented. I think all of Rolf Kuehni's books (Color, Color Space and its subdivisions, and Color Ordered) have good summaries of the development of the Munsell system, with links to the primary literature.

Thanks by the way for your efforts in the battle to distinguish chroma and saturation!

Regarding chroma and the rest, I don't think my efforts were very explicit or clear, unfourtunately - you got what I meant because, of course, you already know it far better than I do.

My bet is that after the renotation (Slinberg was clearly talking from the after-renotation standpoint) you have a Munsell theory that is qualitatively different from the original one. A far more reliable one, but I think you gain that at the expense of something else. It becomes sort of CIE with perceptually uniform re-scaling(?). The mistery of the hue divisions is part of what fascinates me, and I'd like to confirm if I'm right about the least-difference value scale (or if it was a middle-value one), or what I think I got regarding the use of the color-tops (instead of photometry). I'm really curious about that paper.

Tyler,on recollection, I have to concede that I get pretty emotional on some color theory aspects - just mention Goethe, or worse, that Itten fellow, and I loose all composure! :) That's because of the years of frustration from getting bad advice from (well-meaning, and otherwise competent) art teachers. So yeah, it's not the same as those vague "what is art" themes, but it can get pretty hot headed.

This has been an amazing series, with some great info and dialogue. In response to Tyler’s observation, and a wink to Antonio, let me add a little counterpoint to this very serious discussion.

So… These three Primary Colors walk into a bar, and the brightest one says: I’m feeling blue… my value is so low, I might be mistaken for a purple if I don’t lighten up. They had just come from the Color Academy, and were looking for a place to cool down a bit. Just then this gorgeous Magenta sidles up to them and says: You Hues are looking a little toned down this evening, mind if I stimulate your retina and see what kind of after-image arises? The secondary Color turned green, but the tertiary was beginning to enjoy the complament, finding it all a little tintilating, he responded with the highest frequency he could manage, (he was beginning to feel a little saturated, and even this ‘cool red’ was starting to look quite chromatic in the dim light) ‘Your looking like a primary color to me, and I’m not cyan!’ ‘Orange you lucky my boyfriend Infra-Red can’t be seen here, he has an ultra-violet tempera.’ She responded. The rest of the colors took this as their cue to fade, and became quite fugitive. On their way out they both looked up at the sign over the door with a shudder: ‘Goethe’s Bar and Grill, all you can Itten.’